1. Field of the Invention
The present invention relates to a solenoid-operated pilot valve for controlling a main valve between open and closed states, and more particularly to a normally open type solenoid-operated pilot valve comprising a pilot hole formed in a main valve member which pilot hole is opened and closed by a pilot valve element magnetically coupled to a solenoid to control said main valve by pilot operation between open and closed states.
2. Discussion of the Related Art
There are two types of main valves having a solenoid-operated pilot valve (normally open type or normally closed type). With the solenoid de-energised in the first type the main valve is open, in the second case the main valve is dosed. A normally open type valve is disclosed in applicant""s own unexamined Japanese patent publication (KOKAI) 11-230398 (filed Feb. 20, 1998, published Aug. 27, 1999). In said valve (FIG. 7) an annular valve element facing the valve seat is secured to a main valve holding cylinder defining a main valve member. An iron core is coupled integrally to said main valve holding cylinder and is axially movably received in a sleeve for common movements with said main valve member. Along an axis of said fixed iron core said pilot hole is formed with an end opening located in said sleeve. Said pilot hole communicates with the interior of the pressure control chamber. The other end of said pilot hole is open to said low-pressure side fluid passage. At a location behind said fixed iron core at the opposite side of said main valve member a movable iron core is axially movably arranged in said sleeve. A pilot valve member is attached to an end face of said movable iron core such that said pilot valve element faces the opening of said pilot hole in said fixed iron core. Both iron cores are in mutual engagement by means of a coil spring. The freely expanded length of said coil spring keeps a predetermined distance between both iron cores without electromagnetic force acting upon the movable iron core. In the de-energised state high-pressure fluid reaches said pressure control chamber through a leak hole formed in said main valve holding cylinder. The fluid further passes to said low-pressure side fluid passage through a gap between the lower iron core and the sleeve and finally through said pilot hole. Since with said pilot valve open the fluid cannot build up considerable pressure said main valve member is pushed upwardly by the primary pressure of the introduced fluid and thus opens. In the energised state the movable iron core is attracted to the fixed iron core such that the pilot valve member doses the pilot hole. The pressure in the pressure control chamber rises due to fluid leaking in from the high-pressure side fluid passage through the leak hole. A pressure difference is built up between said pressure control chamber and the low pressure side fluid passage moving said main valve holding cylinder towards said valve seat. Said main valve element is pressed against the valve seat and remains stationary in the closed state of the main valve. If in this close state the coil is de-energised, the movable iron core inside said sleeve is pushed back under the urging force of the coil spring until both iron cores are separated by said predetermined distance. The pilot valve is opened and the pilot hole communicates with the low-pressure side fluid passage. The pressure in the pressure control chamber decreases while the pressure of the high-pressure side fluid passage acts upon the main valve holding cylinder. A pressure difference is built between the high-pressure side fluid passage and the pressure control chamber. Due to said pressure difference the main valve holding cylinder moves in a direction such that the main valve element is moved away from said valve seat and opens said main valve. If the main valve suddenly opens while simultaneously high pressure fluid is introduced to the high-pressure side fluid passage said main valve element is deformed by the pressure introduced into a gap between the main valve member and the valve element. Said valve element even may come off said main valve member. To prevent this a pressure relief hole is formed through said main valve member such that pressure introduced into said gap between said main valve member and said valve element can escape avoiding undesirable deformation of said valve element and tearing off said valve element from said main valve member.
Since said main valve holding cylinder and said fixed iron core are integrally coupled to each other and are designed to simultaneously slide within a cylindrical hole of the housing and said sleeve, respectively, there is a danger that the main valve holding cylinder and the fixed iron core get out of mutual alignment. This leads to the problem that the valve in operation cannot perform smoothly. If the fixed iron core received in said sleeve is inclined or gets out of alignment with the sleeve, a front surface of the fixed iron core facing the pilot valve element coupled to the movable iron core fails to come into close and correct contact with the pilot valve element uniformly and over the entire circumference when the pilot valve to be closed. This causes an internal leak. Fluid passing from said high-pressure side fluid passage via said leak hole, said pressure control chamber and said pilot hole in the fixed iron core reaches the low-pressure side fluid passage through the gap provided the fixed iron core and the sleeve. i.e. along the sliding portion of the fixed iron core. Foreign matter contained in the fluid enters the gap and causes a problem, because then the movable iron core does not slide properly. In order to prevent the valve element from coming off the main valve member in case of a switchover of high-pressure fluid said main valve member needs to be drilled to form a pressure relief hole. Said disadvantages lead to a deterioration in operation performance due to a misalignment of the pilot valve element and a deterioration in sliding performance due to foreign matter. Drilling a pressure relief hole is an additional costly manufacturing step.
It is a task of the invention to provide a valve as disclosed above which is fail-safe in view to misalignments of the pilot valve and in the sliding performance due to foreign matter contained in the fluid and which does not need drilling a pressure relief hole for the operational safety of the valve element.
In the valve designed according to the invention the plunger contacts the main valve member when the coil is de-energised while a position of the shaft is maintained in which the pilot valve element is separated from the pilot valve seat such that said pilot valve cannot be closed inadvertently. If fluid is entering from the high-pressure side fluid passage said fluid does not act upon the main valve member in closing direction of the main valve since the pressure control chamber is open to the low-pressure side fluid passage via said open pilot valve. The fluid instead acts upon main valve member in opening direction so that the main valve opens. As soon as the coil is energised the plunger is attracted to the movable core. By displacement of said shaft relative to said plunger said pilot valve element protrudes from the plunger and safely closes said pilot valve. The pressure in the pressure control chamber rises due to fluid leaking through from the high-pressure side fluid passage. A pressure difference is built up with the result that the main valve is moved towards the valve seat and is closed and kept in its closed state.
According to the invention said plunger is structurally separated from said main valve member. Said separation prevents a deterioration in operation performance in case of a misalignment between the plunger and said main valve member. Fluid flowing into the pilot hole from said pressure control chamber does not flow along the sliding portion of the plunger, but instead directly into the plunger and towards the pilot valve seat. This guarantees a perfect sliding performance of the plunger, since no foreign matter can deteriorate the sliding performance of the plunger.
According to a further aspect of the invention the main valve member has the pilot valve seat and said valve element attached separately from one another. Said valve element is trapped by caulking at its outer periphery and inside the recess of said main valve member. A gap is defined at the inner peripheral portion of said valve element and serves as a pressure relief passage. For that reason it is not necessary to form an additional pressure relief hole in said main valve member.